The availability of lightweight plastics has led to the replacement of glass by such plastics for numerous uses. Over the past several years, new plastics have been developed which find use in window glazing, lenses, clear face shields, aircraft canopies and the like. Although such plastics have many outstanding properties, they are deficient in their resistance to scratching. An outstanding example is the deterioration of plastic sunglass lenses by common everyday use because such glasses are frequently removed from the face and laid on hard substrates, lens down.
Thus, in order to better utilize the advantageous properties of today's plastics, there is a need to render such plastics scratch and abrasion resistant.
In order to obtain abrasion resistant surfaces, such as, for example, polycarbonate surfaces, investigators have tended to coat very thin coats of organic or silicone resins on the surface of the plastics. The intent was to obtain abrasion resistance without losing the optical properties of the plastic substrate.
Such an organic coating is disclosed in U.S. Pat. No. 4,018,941. Such an organic coating is prepared from polyols and urethanes and is cured via melamine crosslinkers. Although some degree of abrasion resistance is afforded by the melamine coating, it has a tendency to be affected by outdoor exposure and eventually the coating deteriorates.
In view of the above, silicone coatings which have a tendency to be very weather resistant were developed. Such resins are shown in U.S. Pat. Nos. 3,389,114, 3,389,121, 3,634,321, 3,642,698 and 3,935,346, all assigned to Owens-Illinois. The latter patent teaches a method of making an abrasion resistant coating from an alkylated melamine-formaldehyde resin and a hydrolyrate of MeSi(OR).sub.3. These resins all have good weather resistance but rather moderate abrasion resistance.
There was developed later on a siloxane resin having a low degree of organic substitution. The coating had hardness and therefore good abrasion resistance. Such resins are disclosed in U.S. Pat. No. 3,986,997 issued Oct. 19, 1976 to Harold A. Clark and assigned to the Dow Corning Corporation. The Clark resins are very versatile materials and find utility as abrasion resistant coatings on a number of substrates which require good abrasion resistance.
The only disadvantage of the Clark resins is the fact that they tend to be inflexible, that is, under certain circumstances the coatings tend to craze.
It is well known in the silicone art that flexibility can be built into a siloxane resin coating by merely incorporating a dimethyl containing hydrolyzable silane in the formulation when the resin is first prepared (see Canadian Pat. No. 1,015,888). Unfortunately, the presence of dimethyl siloxane in any siloxane resin tends to also soften the coating so that the abrasion resistance falls off. Thus, for purposes of obtaining an abrasion resistant coating with flexibility, one would not suggest using the above approach in preparing the resins.
What is needed is a weather resistant, abrasion resistant, flexible, clear coating.